Radar level gauge systems are today in use in a variety of fields of application for accurate filling level determination. For determining the filling level of a product by means of a radar level gauge system, electromagnetic signals are transmitted and propagated, usually by means of an antenna, towards a surface of the product, where signals are reflected. The reflected signals are received by the radar level gauge system, and the distance between a reference position and the surface of the product is determined by comparing the transmitted signals with the reflected signals. Based on this distance, the filling level can be determined.
Most radar level gauge systems on the market today are either systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type. There are also radar level gauge systems that are a mix between the two types mentioned above.
Different radar level gauge systems can determine the filling level of a product contained in a tank with different sensitivity levels. Generally, in a radar level gauge system having a high sensitivity, the filling level of the product in the tank can be determined even if a very weak electromagnetic signal is transmitted. The weakest transmitted signal enabling filling level determination may be used as a measure of the sensitivity of the radar level gauge system.
Depending on the field of application of the radar level gauge system and factors such as the electrical properties of the product contained in the tank, there are different requirements on the sensitivity of the radar level gauge system. For example, a high sensitivity is generally required to ensure a reliable filling level determination when the product inside the tank has similar signal propagation characteristics as the atmosphere in the tank. This results in a small impedance transition at the surface and, accordingly, a relatively weak echo signal. Examples of products yielding relatively weak echo signals are liquid natural gas (LNG), liquid petroleum gas (LPG), oil-based products, solids such as plastic pellets or grain etc.
Furthermore, a high sensitivity is generally required in open or semi-open applications, such as open tanks, floating-roof tanks, reservoirs, rivers, or lakes, when the operation of the radar level gauge system is typically subject to government regulations in respect of such parameters as the frequency and power of the transmitted signals.
In view of demanding applications such as those exemplified above, currently available radar level gauge systems often exhibit an insufficient level of sensitivity.